1. Field of the Invention
The present invention relates to a semiconductor device having a circuit comprising a thin film transistor (TFT) that uses a crystalline semiconductor film formed on a substrate (a liquid crystal display device, in particular), and to a method of manufacturing the semiconductor device. The semiconductor device manufactured in accordance with the present invention is specifically a liquid crystal display device represented by an active matrix liquid crystal display device in which a pixel portion and a driver circuit to be placed in the periphery of the pixel portion are formed on the same substrate. The invention also relates to electronic appliances that employ the display device as a display unit.
2. Description of the Related Art
TFTs with a crystalline semiconductor film (typically, a polysilicon film) on an insulating surface as a semiconductor element are used for various integrated circuits at present. The TFTs are used most often as switching elements of a display device. The TFTs having, as an active layer (a semiconductor layer including a channel formation region, a source region, and a drain region), a crystalline semiconductor film, which provides higher mobility than an amorphous semiconductor film, are high in driving performance, and hence used also as elements of a driver circuit. Accordingly, in an active matrix liquid crystal display device, for example, an image circuit for displaying an image and a driver circuit for controlling the image circuit are formed on a single substrate.
In an active matrix liquid crystal display device, integrated circuits such as a pixel circuit for displaying an image, a shift register circuit based on a CMOS circuit, a level shifter circuit, a buffer circuit, and a sampling circuit are all arranged on a single substrate while forming different functional blocks. A liquid crystal display device as above has excellent features including being thin, small-sized, light-weight, and low in power consumption. For that reason, the liquid crystal display device is now used in various scenes; to name a few, as a display unit of a personal computer for space saving and as a display unit of a portable information equipment for obtaining the latest information anytime, any place.
A pixel portion of the liquid crystal display device has a TFT functioning as a switching element (also called a pixel TFT and a storage capacitor, and is driven by applying a voltage to a liquid crystal. The liquid crystal has to be driven with an alternate current, and a method called frame inversion driving is often employed. The TFT is required to have a characteristic of sufficiently low OFF current (Ioff: the value of drain current flowing when the TFT is in off-operation). However, OFF current is often high when the TFT is formed of a polysilicon film. A known solution for this problem is the LDD structure with a low concentration impurity region (lightly doped drain) (a structure in which an impurity region having a low concentration is provided between a channel formation region and a source region or a drain region doped with a high concentration of impurity element).
On the other hand, high driving voltage is applied to a buffer circuit and the circuit needs to have a withstand voltage high enough to prevent damage against high voltage. In order to enhance the current driving ability, the ON current value has to be sufficiently high (Ion: the value of drain current flowing when the TFT is in on-operation). Degradation of the ON current value due to hot carriers is effectively prevented by a known structure called the GOLD (gate-drain overlapped LDD) structure in which a gate electrode partially overlaps an LDD region (with a gate insulating film interposed therebetween).
In order to obtain a semiconductor device that meets the required performance, it is necessary to fabricate different TFTs for different circuits. However, increased number of masks are needed to form an LDD structure TFT and a GOLD structure TFT. An increase in number of masks used leads to more manufacture steps, complication of the manufacture process, and reduction in yield.